This invention relates to a valve operating system for an engine and more particularly to an improved method for forming the camshaft surfaces thereon for operating a plurality of valves from a single camshaft where the valves reciprocate about axes that are skewed to each other.
In order to provide an optimum combustion chamber configuration and large flow areas, it has been proposed to provide a cylinder head wherein at least two valves are operated by the same camshaft for a given cylinder and these valves reciprocate about axes that are not parallel to each other but rather are in a skewed relationship. By employing such an arrangement, it is possible to form a combustion chamber shape that is more like a segment of a sphere.
However, this gives rise to considerable difficulties in the actuation of the valve. That is with conventional camshaft operated valves, the valve reciprocates about an axis that lies in a plane that is perpendicular to the axis of the camshaft. The valve reciprocal axis may also intersect the camshaft axis, but this is not essential. If the valve reciprocal axis does not lie in a plane that is perpendicular to the camshaft axis, it is difficult if not impossible to operate it without scuffing between the valve and its actuating element.
A system has been proposed wherein each valve is operated by a pair of rocker arms. The first rocker arm is operated by a respective cam lobe on the camshaft. This first rocker arm operates the valve through the second rocker arm. The rocker arms are supported so that their pivotal axes are at an angular relationship to each other so as to minimize sliding or scuffing contact between the rocker arm that operates the valve and the tip of the valve stem.
Obviously, the use of such multiple rocker arms substantially complicates the engine construction and minimizes the available space for other components in the cylinder head arrangement.
Therefore, there has been proposed a system wherein the valve is operated by a single rocker arm that contacts a three-dimensional cam surface formed on the camshaft. With this type of arrangement, only one rocker arm need be employed for each valve. However, it is important that the contact between the cam surface and the rocker arm be maintained at a line contact rather than a point contact. If this is not done, there will be a substantial problem because the oil film will break down at the point of contact between the cam and its follower surface.
It is, therefore, a principal object of this invention to provide an improved cam and follower arrangement for operating a poppet valve through a three-dimensional cam surface of the camshaft.
It is a further object to configure the cam surface in such a way that it has a curvature that will be mated with a curved surface of the follower of the rocker arm so as to maintain a line contact during the opening and closing of the associated valve.
It is a further object of this invention to provide an improved method for forming a cam lobe that will achieve the results desired.
This invention is adapted to be embodied in a method for forming a camshaft for a valve actuating system for an internal combustion engine wherein the camshaft has a cam lobe formed thereon that has a three-dimensional configuration. A rocker arm has a follower surface that is engaged with this cam lobe for pivoting the rocker arm about a pivotal axis that is disposed at a skewed angle to the axis of rotation of the camshaft. The rocker arm has an actuating surface that is engaged with a poppet-type valve that reciprocates about an axis that is skewed relative to the camshaft axis and which lies in a plane that is generally perpendicular to the rocker arm pivot axis. The cam lobe is configured so as to have a slight concavity on its follower engaging surface so as to maintain a line contact with the curved follower surface of the rocker arm.
The invention is embodied in a method for forming a cam lobe having a configuration as described in the preceding paragraph. This method involves the mounting of a camshaft blank having a blank cam lobe for rotation about an axis that is coincident to the rotational axis of the camshaft in the engine. A grinding wheel having a curved grinding surface is mounted for rotation about an axis that is skewed to the axis of rotation about which the mounted camshaft rotates. This axis is also supported for translational movement relative to the camshaft axis for forming the desired camshaft lobe configuration. The camshaft is rotated slowly about the camshaft axis while the grinding wheel is rotated about its axis. This axis is translated relative to the camshaft axis so as to form the desired cam lobe configuration. The movement of the grinding wheel axis is done in a manner so that the grinding wheel will contact the cam lobe surface along a line that conforms to the line of contact of the cam lobe with the follower when mounted in the engine.